In the manufacture of integrated circuits each element is ordinarily produced by a lithographic process wherein an image of one or several of the individual circuit elements or features is projected onto the wafer at a time. Because of the small size of such elements and the importance of their being accurately sized and positioned, the fabrication equipment is frequently checked for a number of possible dimensional anomalies and aberrations. These checks include "best focus" at the center of the lens for both the x and y directions as well as the "best focus" at a number of locations displaced from the center, again for both the x and y directions. Likewise, it is checked for coma (comatic aberration) and other aberrations. Such aberrations are well known in the art. See, for instance, E. HECHT, OPTICS (2d ed. reprinted with corrections 1990) p.223 or OPTICAL SOCIETY of AMERICA, HANDBOOK OF OPTICS Vol. II (2d ed. 1995) Chapter 1.
The usual way at the present time for making such checks and measuring discrepancies is with a traditional box in a box configuration having an outside box of about 20 .mu.m square defined by a line or trench and a centrally disposed inside box of about 10 .mu.m square similarly defined by a line or trench. Sometimes, one of the boxes is formed as a solid square rather than merely a line configuration. Moreover, the boxes need not be limited to clear tone patterns but could be formed of a dark tone. In any case the square in a square configuration utilizes a measurement between edges of the two squares. Such measurements can be taken with the aid of a scanning electronic microscope (SEM) and can be very accurate when measuring relatively large areas or verifying the position and dimensions of relatively large features. Even so, they may be quite time consuming since only about 40 individual measurements can be completed per hour.
Not all features of integrated circuits are in the relatively large category. In many instances, some of the individual features of the circuit, such as the gate lines, have extremely small dimensions and may have widths of less than 0.2 .mu.m to 0.4 .mu.m with their lengths being considerably greater, perhaps 0.8 .mu.m to 2.0 .mu.m. When dimensions reach such small size there is a tendency for the formed line to be shorter than its design length as defined by the mask. Such "line shortening", as the term is used herein, occurs when a line is particularly thin because the light passed through the mask to define the periphery of the line, while relatively constant along the length of the line, is amplified at each end of the line due to the fact that the end of the line is subjected not only to the light along the two sides but also the light at the end of the line. The shortening effect is increased when the projection is out of focus, thereby causing more light scatter. Consequently, the focus throughout the entire field of the projection must be considered. In addition the length of the line may be reduced by coma and this aberration is also carefully monitored. In such cases, the accuracy of the lens projecting light through the particular mask to create the features becomes extremely important.
When the feature is in the form of a long thin line, the usual box in a box system does not accurately measure the length of the line. Rather, it merely measures the distances between the walls of the two boxes thereby ignoring any line shortening. Moreover, measurements of the feature with an SEM is impractical because, in order to place the entire line in the measurement screen, only a very low magnification of about 10K to 20K can be employed. At such magnification the precision is only in the order to 20-25 nm.
Even though the projection of the entire mask onto the wafer has a usual dimension of less than 20 mm square, variations of the lens parameters over that small area can be critical. The slightest aberration in the lens projection can cause errors in the construction of the individual elements such as a fatal reduction in the length of the gate line known as "gate shortening". Thus, it is intended that the focus of the lens lies in a single plane and, if there is any variation from that single plane, it has to be identified so that corrections can be applied. This is important not only for the center of the lens, but for various points throughout the projection area. The same is true for other lens aberrations such as coma which can intensify the reduction in line length.